1. Field of the Invention
This invention relates to a through-type capacitor and a magnetron using the same, and more particularly to a through-type capacitor of high dielectric strength suitable for use for a high-frequency and large-power apparatus such as, for example, an electronic range or cooking stove, a broadcasting magnetron, a noise filter for an X-ray tube or the like and a magnetron in which such a through-type capacitor is incorporated.
2. Description of the Prior Art
A conventional through-type capacitor is typically constructed in such a manner as shown in FIGS. 1 to 3. More particularly, a conventional through-type capacitor which is generally designated by reference numeral 30 in FIGS. 1 to 3 includes a ceramic dielectric 32 formed into an elliptic shape. The ceramic dielectric 32 is formed with a pair of vertical through-holes 34 in a manner to be substantially parallel to each other. Also, the ceramic dielectric 32 is provided on an upper surface with a pair of electrodes 36, which are separated from each other and on a lower surface thereof with a common electrode 38. The separate electrodes 36 and common electrode 38 are formed with through-holes corresponding to the through-holes 34 of the dielectric 32, respectively. The capacitor 30 also includes a ground fitment 40 formed at a central portion thereof with an opening 42 and provided on one surface thereof with an upstand 44 of a suitable height, which is arranged to surround the opening 42. The ceramic dielectric 32 is fixed through the common electrode 38 on the upstand 44 of the ground fitment 40 using suitable means such as soldering or the like.
Further, the capacitor 30 includes a pair of through-conductors 46 each covered with an insulation tube 48 formed of a suitable material such as silicone. The insulation tubes 48 are inserted via the through-holes 34 and opening 42 and the through-conductors 46 each are fittedly secured in an electrode connector 50 fixed on each of the separate electrodes 36 by soldering or the like. Fixing of the conductor 46 with respect to the connector 50 may be carried out by soldering or the like.
The ground fitment 40 is formed of a metal plate by drawing so that the upstand 44 of a suitable height may be formed at an intermediate portion of the one surface of the fitment 40 so as to outwardly project from the fitment and surround the opening 42 and a recess 52 may be provided on the other surface of the ground fitment 40 to provide an inner surface of the upstand 44.
The capacitor 30 also includes an insulation case 54 securely fitted at a lower portion thereof on the upstand 44 of the ground fitment 40 so as to surround the ceramic dielectric 32 and an insulation cylinder 56 securely fitted at an upper portion thereof in the recess 52 of the ground fitment 40 so as to surround the through-conductors 46. The insulation case 54 and insulation cylinder 56 are filled with insulation resin materials 58 and 60 such as epoxy resin or the like to cover an outside and and inside of the ceramic dielectric 32 with the resins or embed it therein, to thereby ensure moistureproofness and insulation properties of the ceramic dielectric 32.
The insulation case 54 and insulation cylinder 56 are formed of a thermoplastic resin material such as PBT or the like. Use of thermoplastic resin exhibits an advantage of absorbing stress due to heat shrinkage of the insulation resins 58 and 60 because it is relatively flexible and shrinkable.
The through-conductors 46 each are integrally formed at an end thereof received in the insulation case 54 with a fastening tab 62 in a manner such that it may be projected from an end of the insulation case 54 so as to facilitate connection of an external connector thereto.
Use of the conventional through-type capacitor constructed as described for a magnetron of an electronic range or the like causes the capacitor to be exposed to high humidity, oil fume, soot, dust and the like, because it is typically operated in a kitchen or the like. In the conventional capacitor, as noted from the foregoing, the fastening tabs 62 to which high voltage is applied and the ground fitment 40 are exposed to an ambient atmosphere, and likewise the insulation case 54 is exposed at an outer surface thereof to an ambient atmosphere. Such construction causes oil fume, soot, dust and the like to be adhered to an outer surface of the insulation case 54 due to electrostatic force produced by application of the high voltage. When moisture condensation occurs due to a variation in temperature of the ambient atmosphere in addition to such a phenomenon, the outer surface of the insulation case 54 is wetted to cause a surface resistance thereof to be highly decreased, resulting in creeping discharge occurring through a passage from the fastening tabs 62 via a surface 64 of the insulation resin 58 and the insulation case 54 to the ground fitment 40. This leads to carbonization of the surface of the insulation case 54 formed of thermoplastic resin to cause the creeping distance to be further shortened, resulting in burning of the insulation case 54.
In order to prevent such burning of the insulation case 54, flame retarder is conventionally added to thermoplastic resin for forming the insulation case 54. Unfortunately, addition of the flame retarder substantially deteriorates tracking resistance and arc resistance of the insulation case 54 to substantially fail to prevent damage of the case due to burning.
As another conventional means for preventing burning of the insulation case 54, it is proposed to form the insulation case 54 of a fire resistant thermosetting resin material or ceramic material. However, this causes the insulation resin or epoxy resin 58 filled in the insulation case 54 to be firmly adhered to an inner surface of the insulation case 54 during a step of curing the resin 58, so that tensile stress is generated in the insulation resin 58 in a direction toward the insulation case 54. This leads to peeling of the insulation resin 58 from an outer surface of the ceramic dielectric 32, resulting in deterioration of dielectric strength of the ceramic dielectric.
The conventional through-type capacitor is typically incorporated in a magnetron in such a manner as shown in FIGS. 4 and 5. A conventional magnetron generally indicated by reference numeral 66 in FIGS. 4 and 5 includes a filter box 68 and a cathode stem 70 having a cathode terminal and arranged in the filter box 68. The magnetron 66 also includes a pair of inductors 72, which are connected to the through-conductors 46 of the capacitor 30. The capacitor 30 is inserted through an opening 74 formed at a side wall 76 of the filter box 68 in a manner such that the insulation case 54 is outwardly projected from the filter box 68 and fixed at the ground fitment 40 to the filter box 68. The inductors 72 are connected in series between the cathode terminal of the cathode stem 70 and the through-conductors 46 of the capacitor 30. Reference numerals 78, 80, 82, 84 and 86 designate a magnet, a cooling fin, a mounting plate, a gasket and an RF output terminal, respectively.
When the conventional magnetron constructed as described above is used for an electronic range also called an electronic cooking stove, the through-type capacitor exhibits such disadvantages as described above to deteriorate satisfactory operation of the magnetron. Also, the construction causes water droplets 88 due to moisture condensation on surfaces of the cooling fin 80 and/or mounting plate 82 to drop on the surface of the insulation case 54 and then penetrate into the case. This leads to wetting of the insulation case 54 to substantially decrease surface resistance of the case to cause such creeping discharge as noted above, resulting in the same disadvantages as described above.
Such disadvantages noteworthily appear when the filter box 68 is formed into a small size to decrease a distance d between the cooling fin 80 and the insulation case 54, because a creeping distance between the fastening tabs 62 and the cooling fin 80 is reduced to a degree sufficient to further facilitate burning of the insulation case 54 due to creeping discharge.
Further, in the conventional through-type capacitor, as described above, each of the through-conductors 46, as shown in FIG. 6, has integrally attached thereto the fastening tab 62. More particularly, the through-conductor 46 is made by bending a section of a blanked metal plate other than a section thereof for the fastening tab 62 to form the through-conductor. The so-formed through-conductor 46 has an angular circle-like shape in section as shown in FIG. 7, thus, the prior art fails to form the through-conductor into a substantially circular shape. Accordingly, when the through-conductor 46 is protected with the insulation tube 48, the whole configuration of the through-conductor 46 and tube 48 is still angular as shown in FIG. 7. This causes stress generated during curing and contraction of the insulation resin 60 to be uniformly distributed as indicated by arrows in FIG. 7 to deteriorate thermal cycle resistance of the capacitor in repeated heating and cooling operation, resulting in a failure in dielectric strength of the capacitor.
Further, the through-conductor 46 is typically subjected to a surface treatment by plating of Sn or the like for the purpose of improving solderability and rust prevention. However, the angular shape of the conventional through-conductor shown in FIGS. 6 and 7 causes an acidic plating solution to remain in the bent portion of the conductor after plating to promote corrosion of the conductor and/or hinders contacting of the bent portion with a plating solution during plating to lead to a failure in plating of the bent portion.
Accordingly, it would be highly desirable to develop a through-type capacitor which is capable of effectively preventing peeling of an insulation resin from a ceramic dielectric and providing an insulation case of the capacitor with heat resistance, tracking resistance, arc resistance to improve burning resistance of the case, resulting in safely and positively operating for a long period of time, and a magnetron having incorporated such an advantageous capacitor incorporated therein.